A high-strength aluminum material has been strongly desired for a vehicular structural member (e.g., side member) and a vehicular energy-absorbing member (e.g., bumper reinforcement and side door beam) in order to reduce fuel consumption through a reduction in weight.
However, when the content of Mg, Zn, Cu, and the like in a JIS 7000 series aluminum alloy is increased in order to improve the strength of the material, a decrease in toughness that has a trade-off relationship with the strength may occur. Moreover, since MgZn2 precipitates having a potential lower than that of aluminum may be produced at the crystal grain boundaries, a deterioration in stress corrosion cracking resistance and a significant deterioration in extrudability may occur.
The applicant of the present application proposed a high-strength aluminum alloy obtained by quenching an aluminum alloy that includes 1.5 to 2.0% of Mg, 7.0 to 9.0% of Zn, 0.2 to 0.4% of Cu, and the like after extrusion at a cooling rate of 1000° C./min or more (see Japanese Patent No. 3735407).
The aluminum material disclosed in Japanese Patent No. 3735407 exhibits high strength and excellent toughness. However, it is necessary to perform water quenching in order to achieve a cooling rate of 1000° C./min or more.
Since the extruded shape may become brittle due to incorporation of hydrogen during water quenching, there has been room for a further improvement in stress corrosion cracking resistance.
As an aluminum alloy that is characterized by the content of Mg, Zn, and Cu, Japanese Patent No. 3834076 discloses an aluminum alloy that is used to produce an automotive constituent member, and includes 0.9 to 1.3% of Mg, 8.0 to 10.0% of Zn, and 0.45 to 0.55% of Cu, and Japanese Patent No. 2928445 discloses a high-strength aluminum alloy that includes 1.0 to 1.5% of Mg, 5.0 to 7.0% of Zn, and 0.1 to 0.3% of Cu. However, since the Mg content in these aluminum alloys is 1.3% or less or 1.5% or less, it is difficult to obtain a 0.2% proof stress of 470 MPa or more.
In Japanese Patent No. 3834076, since the Zn content is as high as 8% or more, a deterioration in stress corrosion cracking resistance may occur.
Japanese Patent No. 4498180 discloses an aluminum alloy that includes 1.9 to 2.6% of Mg, 5.7 to 6.7% of Zn, and 2.0 to 2.6% of Cu. However, since the Cu content is 2.0% or more, it is expected that the extrudability significantly deteriorates. Therefore, the aluminum alloy disclosed in Japanese Patent No. 4498180 is not suitable for producing an extruded shape having a hollow cross-sectional shape (e.g., bumper reinforcement).